1. Field of the Invention
The present invention relates to a power seat driving mechanism comprising a gear mechanism for coupling a source of rotational energy to a threaded shaft which drives the seat to move with respect to a guide rail on which it is mounted. Still more specifically, the instant invention relates to an improved drive mechanism for a power seat wherein backlash in the drive train which drives the threaded member is eliminated.
2. Description of the Prior Art
In automotive vehicles it is common to use power seat mechanisms for driving a passenger seat to a position selected by the seat occupant and thus obviate the need for the occupant to manually exert force on the seat for said purpose. An example of such a power seat mechanism is illustrated in FIGS. 1 and 2.
Referring now to FIG. 1, the overall structure of the prior art power seat driver mechanism is shown in perspective. As will be noted this view includes the structural members of the seat to which the seat driver mechanism is directly coupled and the guide rail by which the seat is coupled to the floor of the vehicle (not shown).
The illustrated prior art seat slider device includes a pair of seat cushion rails 1 which are usually formed integrally with the bottom portion of the frame of the seat cushion (not shown) and the seat cushion frame forms the base structure of the seat. The seat cushion slider rails 1 are formed so as to have generally C-shaped cross sections.
The slider rails 1 are slidably supported on a pair of guide rails 2. In this arrangement the guide rails 2 are partially enclosed by the slider rails 1 and are rigidly mounted to the floor (not shown) of the vehicle so as to be aligned in the fore and aft direction of the vehicle and are parallel to each other. The seat cushion slider rails 1 are thus mounted on guide rails 2 so as to be slidable therealong.
At one end each of the respective seat cushion slider rails 1 are inwardly protruding brackets on which one end of a threaded shaft member 3 is rotatably supported. At the other ends of the seat cushion rails, inwardly protruding brackets are provided for supporting a pair of gear housings 6.
A source of rotational energy, such as an electric motor 5, is supported on one of the gear housings 6. The motor 5 is coupled to a drive shaft 7 so as to be operable to drive the drive shaft 7 to rotate. The drive shaft 7 extends between the gear housings 6 and the ends of the drive shaft 7 terminate within the gear housings 6.
One end of each of the respective threaded shaft members 3 terminates within the gear housings 6. Thus, both the threaded shafts 3 are rotatably supported at both ends so as to extend along the inner sides of the seat cushion rails 1 in a parallel relationship thereto. The shafts 3 are supported in such a manner that, while being rotatable, they cannot be moved longitudinally with respect to the seat cushion rails 1.
At its central portion, the threaded shaft 3 passes through a seat driver nut 4. The seat driver nut 4 is affixed to the floor mounted guide rail 2 or to the floor (not shown) of the vehicle by means of a bracket (not shown). Thus, the seat driver nut 4 is held rigid with respect to the guide rail 2 and cannot move longitudinally with respect thereto.
In FIG. 2, one of the gear housings is indicated in phantom lines so as to reveal the gear mechanism disposed therewithin. It will be understood that the gearing mechanisms in the respective gear housings at the terminal ends of the shaft 7 are essentially mirror images of each other. Accordingly, the one depicted in FIG. 2 is representative of both.
A worm 7a is disposed within the gear housing 6. The worm 7a is rigidly coupled to the drive shaft 7. The thread of the worm 7a meshes with the teeth of a worm wheel 8. The worm wheel is rigidly affixed to one end of the threaded shaft 3.
With the above construction, when it becomes desirable for the seat occupant to adjust the position of the seat with respect to the vehicle chassis in one of the forward or the rearward direction, the motor 5 is energized and drives the drive shaft 7 and the worm attached to the end thereof, to rotate. The rotation of the worm 7a serves to bring a driving surface of the worm 7a into contact with the teeth of the worm wheel 8.
This rotation of the worm wheel 8 is transmitted to the threaded shaft member 3.
The rotation of the threaded shaft member 3 induces it to thread its way through the seat driver nut 4.
Since the threaded member 3 cannot move longitudinally with respect to the seat cushion slider rails 1, the seat cushion slider rail 1 is driven by threaded member 3 to move longitudinally with respect to the guide rails 2.
In this manner, the position of the seat mounted on the slider rails 1 can be adjusted with respect to the vehicle chassis.
In the above arrangement however, there is a disadvantage of a possibility that a backlash will be generated in the worm gear mechanism. This backlash tends to occur when, the motor 5 is energized so as to alter the position of the seat, the seat occupant manually exerts an additional force on the seat in the direction in which it is being driven, in an attempt to force the seat to move more quickly than it is being driven by the driver mechanism.
Under such circumstances, the additional force exerted on the seat by the seat occupant is translated by the engagement between the threads of the threaded shaft member 3 and the seat driver nut 4 into a tangential force acting on the threaded member 3 in the direction of its rotation. This causes the threaded member 3 to rotate faster than it is being driven to rotate by means of the engagement between the worm wheel 8 and the worm 7a. When the rotation rate of the threaded shaft exceeds the rotational speed at the worm wheel is being driven by the worm, the teeth of the worm wheel tend to disengage slightly from the driving face of the thread of the worm 7a and may even come into contact with the trailing face of the thread of the worm 7a.
Under the above conditions, since the motor 5 is no longer operating under a load, it tends to accelerate and drive the worm 7a to rotate faster. Thus, the worm may accelerate so as to catch up with the worm wheel and the drive face of the threads thereof tends to strike against the driven face of the tooth of the worm wheel 8 which is currently in position to mesh with the worm. This produces a jolt in the rotation of the threaded shaft member 3 which is sufficiently strong as to be perceptible to the seat occupant and cause noise and vibration. This is distracting and annoying for the seat occupant.
What is more, such shocks between the engaging faces of the driving gear of the power seat driving mechanism tend to increase the wear on the gears and the support bearings of the drive shaft 7 and the threaded shaft 3. This in turn, tends to increase the amount of play in the worm gear mechanism, which again increases the rate of wear on the gears 7a and 8 and increases the amount of annoying vibrations that are produce in the seat of the vehicle by the drive mechanism.